Treatment of wood



Patented Nov. 17, 1942 TREATMENT OF WOOD Hamllne M. Kvalnes, Wilmington, DeL, assignor to E. I. du Pont de Nemours a Company, Wilmington, Del., a corporation of Delaware No Drawing. Application August 22, 1940, Serial No. 853,697

Claims.

This invention relates to the treatment of wood and more particularly to the chemical seasoning of wood.

In the lumber industry, one of the most serious problems has been that of checking and cracking of wood during its storage and drying after cutting of the timber. This checking and cracking of wood is due, primarily, to the shrinkage resulting from drying of the outer area or portions of green wood at a more rapid rate,than the interior portions. As a consequence of this uneven shrinkage, surface strains and stresses are set up which cause extensive cracking, checking or splitting of the green lumber, being most apparent at the ends and along the surface of the lumber in the central portions of the flat grain cuts.

Various proposals have been made to overcome these problems but heretofore none have proven commercially practicable.

It is an object of this invention to overcome the drying problem in the lumber industry and to provide a new and improved method for treating or seasoning lumber.

Other objects and advantages of this invention will be apparent from the following specification in which its preferred embodiments and details are described.

I have found that the checking and cracking of unseasoned wood may be greatly reduced by treating wood with a monoammonium phosphate solution of such concentration, at the particular temperature employed. that at the wood surface the monoammonium phosphate solution is completely saturated. Preferably the wood temperature is at a temperature in the range of 5 to C. below the temperature of the monoammonium phosphate solution, and the monoammonium phosphate solution is saturated at the temperature at which it is utilized.

The process of this invention is applicable to wood generally, whether unseasoned or partly seasoned or, in fact, if the wood contains any moisture which it is desired to remove. Thus, for example, the water content of green wood, based upon the dry weight of the wood, may vary from about 30 to 200% or more. Such unseasoned wood may be stored, however, and consequently air-dried, prior to the treatment of this invention so that its moisture content may be reduced to as low as 15 to In the case of such partly seasoned wood our process may 'be applied to remove the greater proportion of the remaining water content. This is followed, preferably, by kiln drying. In normal operation, however, the original water content of wood is reduced by treatment with monoammonium phosphate, followed by air or kiln drying of the phosphate-treated wood. v

The amount of monoammonium phosphate absorbed on the wood surface by the process of this invention is increased greatly over what would be expected, based upon the monoammonium phosphate concentration. Upon removal of wood from the saturated monoammonium phosphate solution a mat of monoammonium phosphate in the form of a fine, intertwined, closely adhering precipitate of crystallized monoammonium phosphate is formed upon the surface of the wood encasing relatively large amounts of monoammonium phosphate solution contiguous to the wood surface. The crystallized monoammonium phosphate mat adheres so strongly to the wood that it is not readily removed by brushing or chipping from rough cut lumber. This mat of monoammonium phosphate subsequently dissolves in the water content of the wood, thus further impregnating the wood surfaces.

Although the dipping of the wood may vary over a wide time interval I have found that within 5 to 30 seconds treatment is highly effective, with a period of 10 to 15 seconds being the preferred time interval.

By the process of this invention, wood surfaces are maintained pervious to the passage of moisture from within the wood so that the interior moisture may continuously be drawn to and evaporate from the outer surfaces. As a result, the surface of lumber-treated according to this invention remains relatively moist while drying proceeds, water inside the wood travels to the outside, thus drying from the inside out rather than from the outside in, and the surface strains and stresses ordinarily present'are reduced to an extent that no appreciable checking or honeycombing occurs.

When wood is dipped with monoammonium phosphate solutions, saturated or unsaturated, near room temperature, the. amount of monoammonium phosphate absorbed on the wood is roughly proportional to the concentration of monoammonium phosphate in the dipping solution. That is also true for under-saturated monoammonium phosphate solutions at temperatures above atmospheric where concentration is not greater than about 25% (the saturation amount at approximately 20 0.). However, when the temperature of the wood is at least 5l5 C. below the temperature of the saturated monoammonium phosphate solution, the amount.

of monoammonium phosphate absorbed on the wood surface is greatly increased over 'what would be ,expected. The action of monoammonium phosphate solutions would indicate that hot saturated water solutions of many other materials which are 'more soluble at temperatures above atmospheric than at atmospheric temperature would behave in the same manner. However, testing of a number of such materials which solution at 60 would be expected to act similarly to monoammonium phosphate, showed that the behavior of monoammonium phosphate to be outstanding. Small differences could be expected due to differences in density, wetting characteristics and viscosity but the amount of chemical absorbed would be expected to be roughly directly proportional to the concentration of the chemical. n the basis of these considerations, the pounds of chemical absorbed per 1000 board feet of a given cross section per weight per cent. of. chemical in the dipping solution should be roughly constant for all materials and at all temperatures.

Experimental evidence shows, however, that for less than saturated solutions, variation from the average for the above ratio is about plus or minus 25%. Experimental results on saturated solutions of the s'agne substances fall in line with the results on les'sthan saturated solutions except for monoammonium phosphate. Absorption of monoammonium phosphate from saturated C. was 64% greater than would be expected from its absorption at 20 C. In contrast, increase in absorption for sodium chloride was 12% and for potassium formate was 19%. These generalities are specifically set forth in the following Tables 1 and 2.

In carrying out the experiments recorded in Tables 1 and 2, green oak blocks 1" x 5" x at ordinary temperature C.) were immersed for 5 seconds in a tank of the indicated chemical solution at 20, 40 and 60 C. as indicated in the tables. The blocks were then removed from the solutions in a horizontal position, allowed to drain 5-seconds at an angle of and then placed horizontally to finish draining. The weight of each block was determined after 5 minutes and the increase in weight taken as the amount of solution absorbed.

From the above tables and considering the extensive experimental work which I have carried on in treatment of wood according to the process of this invention it appears that the magnitude of the monoammonium phosphate deposition and absorption effect is approximately directly related to the temperature difference between the wood and the solution and thatthis process is not greatly afiected by the moisture content of the wood.

Various changes may be made in the preferred embodiments and details of this invention without departing therefrom or sacrificing the advantages thereof.

I claim:

1. In a process for seasoning moisture-containing wood, the step which comprises bringing the outer surface of such wood into contact with a saturated monoammonium phosphate solution, the wood being at a temperature at least 5 C. belowthat of the saturated monoammonium phosphate solution.

2.--In a process for seasoning moisture-containing wood, the step which comprises bringing the outer surface of such wood into contact with a saturated monoammonium phosphate solution, the wood being at a temperature 5-15" C. below that of the monoammonium phosphate solution.

3. In a process for seasoning moisture-containing wood, the step which'comprises bringing the outer surface ofsuch wood into contact for from 5 to 30 seconds with a saturated monoammonium phosphate solution, the wood being at a temperature at least 5 C. below that of the monoammonium phosphate solution.

4. In a process for seasoning moisture-containing wood the step which comprises bringing the outer surface of such wood into contact for 5 to 30 seconds with'a saturated monoammonium phosphate solution, the wood being at a temperature 5 to 15 C. below that of the monoammonium phosphate solution.

. TABLE 1 Absorption of chemical solutions on .o'ak

Conditions Per cent complete satn; oi I chemical in solution 90 90 100 100 C. C'. "0. C. C. C. Temperature of solution... 20 40 60 20 40 00 Lbs. chemical absorbed per 1000 Chemlml board feet of 2" x 12" Sodium chloride 8. 7 8 9 9 l0 Monoainmonium p h o sphate 1 1a 20 9 18 25 Potassium format- 30 46 39 45 48 57 Conditions Per cent sat'n. of chemical in solution 00 90 90 100 100 100 C'. 90. C'. C. "8'. C Temperature-oi solution"; 20 20 60 20 40 60 Wt. per cent chemical in' dipping cpemwal solution Sodium chloride I 24 24 24 27 27 21 Monoammonium phosphate 24 32 41 27 36 46 Potassium formate 69 72 74 77 80 82 5. In a process for seasoning moisture-containing wood, the step which comprises bringing the outer surface of such wood into contact with a saturated monoammonium phosphate solution, the wood being at a temperature at least 5 C. below that of the saturated monoammonium phosphate solution, and thereafter drying the treated wood.

HAMLINE M. KVALNES. 

